Method for forming bicrystalline specimens



Dec. 31, 1963 R. B. POND METHOD FOR FORMING BICRYSTALLINE SPECIMENS 2Sheets-Sheet 1 Filed Jan. 27, 1958 INVENTOR foafmia pan/0,

BY Y4 ATTORNEYS Dec. 31, 1963 R. B. POND 3, 75

METHOD FOR FORMING BICRYSTALLINE SPECIMENS Filed Jan. 27, 1958 2Sheets-Sheet 2 BY a w ATTORNEYS United States Patent 3,116,175 METHGDFOR FURMKNG EECRYSTALLIWE SPEClMlENfi Robert B. Pond, Westminster, Md,assignor to Marvalaud, Inc, Westminster, Md, a corporation of MarylandFiled Jan. 27, 1958, Ser. No. 711,499 6 Claims. ((31. EMS-1.6)

This invention relates to a method for forming bicrystalline specimensand more particularly to a method for producing specimens containing twoor more crystals wherein the grain boundary between the crystals isdisposed substantially transversely to the longitudinal axis of thecrystals.

There are various well-known methods which have been utilized heretoforefor forming single crystals of metals. One prior art method which isknown as the Czochralski method involves lowering a seed crystal into amolten bath of the metal and withdrawing the seed crystal at a specifiedrate and maintaining the seed and the attached material at a temperaturelower than the melting point of the metal. This method produces a singlecrystal extending longitudinally from the seed crystal along the axis onwhich the crystal has been moved. Another wellknown prior art method forforming single crystals which is known as the Bridgeman method involveslowering of a tube which is filled with molten metal through a thermalgradient so that at one point in the travel of the tube the temperatureof solidification will be passed and all metal below this point will besolidified. If this method is carried out by moving the tube at theproper rate and maintaining the appropriate thermal gradient singlecrystals will be formed.

Either the Czochralski or Bridgeman techniques may be utilized toproduce bicrystalline specimens in which the grain boundary between thecrystals is parallel to the specimen axis. This is accomplished by usingtwo seed crystals which are oriented in different manners and carryingout the usual prior art methods. However, it is difiicult, if notimpossible, to produce bicrystalline specimens in which the grainboundary is perpendicular to the specimen axis by utilizing theCzochralski or Bridgeman methods. It would be necessary, using suchtechniques, to seed the molten metal during the growth of one crystalwith a second crystal. It is possible to develop bicrystalline specimensby seeding from two ends of a horizontal container with properlyoriented seeds disposed in each end of the container nad permitting thecrystals which develop to grow together. However, such a method developsunsatisfactory bicrystalline specimens in that a shrinkage cavity isformed at the grain boundary and also excessive impurities are formed inthe area adjacent to the grain boundary. This is due to the fact thatmany of the impurities have a lower melting point than the crystallineelement and consequently the impurities will remain in the molten metaland collect adjacent the grain boundary. Hence, prior art methods havebeen generally unsatisfactory in producing bicrystalline specimens inwhich the grain boundary is disposed substantially transversely to thelongitudinal axis of the crystal.

Bicrystalline specimens which are oriented so that the grain boundary issubstantially transverse to the longitudinal axis of the crystals areparticularly useful in connection with thermocouples and transistors. Ithas been found that any impurity will diffuse more readily at a crystalface than through a crystal and consequently bicrystalline specimens ofgermanium and the like form excellent transistors when indium or boronis diffused across the grain boundary to form the barrier between thedonor and the receptor materials. Furthermore, in connection with thestudy of the physical characteristics of grain boundaries suchbicrystalline specimens have well recognized utility. For example,considerable research has been conducted on the effect of grainboundaries upon the formation of slip planes in stress analysisinvestigations on metals. In this connection reference is made to volume50, Reprint No. 43 of the Transactions of the American Society forMetals in which the grain boundary movement in bicrystalline aluminum isdescribed.

The presently disclosed method for forming bicrystalline specimens ofthe type described involves forming a single crystal by any of the knownmethods, melting a small portion of this single crystal between the endsthereof, rotating one end portion of the crystal with respect to theother end portion and pressing the ends together so that a portion ofthe molten metal is extruded from the body of the crystal. As the moltenmetal solidifies a grain boundary is formed between the differentlyoriented end portions of the specimen.

It could not have been anticipated that such a method would result inthe formation of a bicrystalline specimen having the grain boundarydisposed substantially transversely with respect to the longitudinalaxis of the specimen for the following reasons. It is not possible toapply heat in such a way that the metal is melted along a plane throughthe specimen. Ordinarily it would be expected that fingers orprojections of solid metal would be disposed in the molten portion ofthe specimen. Upon rotation these fingers or projections break 0% and itwould be expected that they would form seeds or randomly disposedcrystals within the melted portion as it solidifies. However, bypressing the ends of the specimen together after rotation these nuclei,if they exist, are extruded from the body of the specimen and thus aclearly defined grain boundary may be formed. In addition, as in thecase of forming bicrystalline specimens with known prior art methods, itwould normally be expected that a shrinkage cavity would be formed asthe molten metal solidifies. This is also obviated by reason of thelongitudinal translation of the specimen after rotation.

An object of the present invention is to provide a method for formingbicrystalline specimens in which the grain boundary is disposedsubstantially transversely with respect to the longitudinal axis of thespecimen.

Other objects and many of the attendant advantages of the presentinvention will become more fully apparent upon consideration of thefollowing detailed specification in connection with the accompanyingdrawing wherein:

FIG. 1 is a longitudinal cross section through a single crystal showinga portion of the crystal in the molten form,

FIG. 2 is a similar longitudinal section showing the specimen in thebicrystalline form, and

FIGS. 3 to 7 inclusive are diagrammatic illustrations of variousembodiments of the present invention.

Referring now more specifically to the drawing wherein like numeralsindicate like parts throughout the several views there is shown at 1 inFIG. 1 a single metallic crystalline specimen. This crystalline specimenmay be formed by any of the well known prior art methods which aredescribed more fully hereinbfeore. Heat is applied on one side of thespecimen between the ends thereof so that a portion 2 is melted. In theFIG. 1 embodiment this melted portion contacts the solid metallic endportions along two angular faces. It may readily be appreciated thatunder ordinary circumstances plane or angular faces would not be formedbut they are shown in this manner for convenience of illustration.

In carrying out the method for forming bicrystalline specimens one endportion of the specimen is held stationary while the other end portionis rotated about the longitudinal axis of the specimen until the endportions 3 and 4 are in the relative position shown in FIG. 2. The endportions are then pressed together so that a portion of the moltenmaterial is extruded from the body of the specimen around the peripherythereof, as shown at 5. By extruding the material many of the impuritiesin the specimen are removed and also randomly disposed seed crystalswhich would tend to form a polycrystalline specimen are extruded. Aclearly defined grain boundary which is disposed transversely to thelongitudinal axis or" the specimen is formed between the end portions 3and 4.

Bicrystalline specimens have been made in which the end portions of thesingle crystal are turned or twisted about axes other than thelongitudinal axis of the specimen. In FIG. 3 a specimen is shown inwhich a grain boundary 6 is formed between the difierently orientedcrystals 7 and 8 and the grain boundary 9 is formed between the crystals8 and 10.

By applying heat substantially uniformly around the periphery of thecrystal a grain boundary which is substantially perpendicular to thelongitudinal axis of the specimen may be formed. In FIG. 4 there isshown a specimen in which the end portions 11 and 11.2 are rotatedthrough 180 about the longitudinal axis of the specimen and the grainboundary is perpendicular to the axis of the specimen. The dotted lineshowings represent similar planes in the end portions of the specimen soas to illustrate the degree of rotation.

FIG. 5 shows a specimen having crystals 13 and 14 in which a 180rotation has been eifected and in which the grain barrier is angularlydisposed with respect to the specimen axis in a manner similar to thatdisclosed in FIGS. 1 and 2.

In FIG. 6 a 90 rotation has been effected and FIG. 7 illustrates a 45specimen rotation. In both FIG. 6 and PEG. 7 a grain boundary isdisposed substantially perpendicular to the specimen annexes.

As a further modification of the present invention it is possible tocombine the Czochralski or Bridgeman methods for producing crystals withthe presently disclosed method to produce a novel crystalline structure.By growing a plurality of crystals from adjacently disposed seeds withthe Czochralski or Bridgeman techniques a multicrystalline specimen isobtained in which the grain boundaries lie in planes parallel to thespecimen axis. Then by carrying out the presently disclosed process onthis specimen through melting a portion of all of the crystals throughthe grain boundaries thereof and rotating one end of the specimen withrespect to the other end a completely novel crystalline specimen isobtained. Each of the single crystals of the original specimen will nowbe bicrystalline and each of the crystals in the specimen will have agrain boundary and another crystal disposed at one end thereof and on atleast one side thereof.

Obviously many modifications and variations of the present invention arepossible in light of the above teachings. What is claimed as new anddesired to be secured by Letters Patent is:

1. A method for forming bicrystalline metallic specimens comprisingforming a single crystal from molten metal, heating to the melting pointa portion of the single crystal across the entire cross-sectional areathereof, rotating one end of the crystal about the longitudinal axisthereof and solidifying the melted portion to form a grain boundary.

2. A method according to claim 1 wherein said one end of the crystal isrotated through 180.

3. In a process of the class described comprising the steps of preparinga single metallic crystal, melting a portion of the crystal across theentire cross-sectional area thereof at a point between the ends thereof,rotating one end portion of the crystal with respect to the other,pressing the ends of the crystal towards each other so as to extrude themolten metal from the body of the crystal and solidifying the moltenmetal to form a grain boundary at the point of rotation of the crystal.

4. In a process according to claim 3 wherein the crystal ends arerotated through 180 about the longitudinal axis of the crystal.

5. In a process according to claim 3 wherein the crystal ends arerotated through about an axis transverse to the longitudinal axis.

6. A method for forming bicrystalline metallic specimens comprising thesteps of forming a single metallic crystal, melting a portion of thecrystal across the entire cross-sectional area thereof, holding one endof the crystal stationary while rotating the other end about thelongitudinal axis of the crystal, pressing one end of the crystaltowards the other end to extrude the molten metal and solidifying themolten metal to form a grain boundary.

References Cited in the file of this patent UNITED STATES PATENTS2,743,200 Hannay Apr. 24, 1956 2,743,201 Johnson et al. Apr. 24, 19562,964,396 Rummel et al Dec. 13, 1960 FOREIGN PATENTS 1,125,277 FranceJuly 9, 1956 742,237 Great Britain Dec. 21, 1955 OTHER REFERENCES Reviewof Scientific Instruments, vol. 25, No. 4, pages 331-334, April 1954.

1. A METHOD FOR FORMING BICRYSTALLINE METALLIC SPECIMENS COMPRISINGFORMING A SINGLE CRYSTAL FROM MOLTEN METAL, HEATING TO THE MELTING POINTA PORTION OF THE SINGLE CRYSTAL ACROSS THE ENTIRE CROSS-SECTIONAL AREATHEREOF, ROTATING ONE END OF THE CRYSTAL ABOUT THE LONGITUDINAL AXISTHEREOF AND SOLIDIFYING THE MELTED PORTION TO FORM A GRAIN BOUNDARY.